Wet oxidation processes for water pollution remediation

Abstract

[eng] The main objective of this work was to test the efficiency of wet oxidation processes when treating several types of aqueous wastes. On one side its performance for the abatement of chloro-organic aromatic toxic pollutants, such as 4-chlorophenol and 2,4-dichlorophenol has been studied. On the other hand, wastewater from pulp and paper mills, which has been reported to be an indirect source of entry of chlorophenols in the aquatic environment, has been investigated. More in detail, it has been taken as feed stream for the wet oxidation unit in order to investigate whether this type of waste streams can be treated by this technology or not. Regarding Chlorophenols, special attention was drawn to the degradation of 4-chlorophenol by means of wet oxidation and wet peroxide oxidation. This aromatic compound was taken into investigation due to its harmful properties against the environment and due to its wide presence in the environment. Once it was clear that it could be degraded by these technologies, a research focused on the influence of the operating conditions in the result of the oxidation was carried out. The influence on the wet peroxide oxidation and wet oxidation reactions of the following parameters, initial concentration of the pollutant, temperature and amount of oxidizing agent (oxygen or hydrogen peroxide depending on the process) has been taken under study. The identification and quantification of the intermediate compounds involved in the wet oxidation of 4-chlorophenol, together with a suggested mechanistic pathway, allowed the obtaining of a kinetic model, which appeared to be a useful tool for the prediction of these compounds throughout the reactions. The evolution of the free chlorine released to the solution from the degraded chlorophenol was also a useful tool when determining the kinetic pathway of the reaction. Another objective of the work comprised the investigation of the variations of the biodegradability of the samples during the process. The knowledge of evolution of this parameter during the wet oxidation was thought to be of major importance, since high biodegradability enhancements allow the combination of a wet oxidation unit with a biological post-treatment, which is an effective and inexpensive technology to couple the oxidation. The establishment of a comparison between wet oxidation and the wet peroxide oxidation for the removal of 4-chlorophenol was investigated as well. Concerning wastewaters from pulp and paper mills, debarking and termo-mechanical pulp process wastewater have been treated by wet oxidation. Both waters were concentrated before oxidation in order to favor the economy of the process. Debarking wastewater was concentrated by evaporation and pulp process water by nanofiltration. The influence of the operating conditions, such as temperature and partial pressure of oxygen, on the results achieved at the end of the wet oxidation were studied and evaluated in order to find the optimum working conditions for each type of wastewater. Special attention was drawn to the evolution of Lipophilic Wood Extractive Compounds throughout the reactions. In addition, kinetic models suggested in the literature were tested to find a suitable one, which allowed the prediction of for instance, the organic load, over the duration of the reactions. Due to the fact that wet oxidation is more economically viable when the initial waste stream is highly concentrated, a final chapter dedicated to a emerging technique, i.e., membrane technology has been included in this thesis. An investigation regarding the parameters affecting its performance, as well as the general aspects of the process has been conducted.